Pressure-roller arrangement for a stacking device of a printer or copier

ABSTRACT

A stacking device in a printer or copier for stacking a strip-like, prefolded recording medium has, as feed arrangement, a motor-driven feed roller (13) and a plurality of adjacently arranged pressure rollers for pressing the recording medium (10) against the feed roller (13). The two outer pressure rollers (16/1, 16/2) in respect of the edge regions of the recording medium are designed as guide pressure rollers which change their inclination of the axes of rotation in dependence on the transporting direction of the paper or recording medium and thus make the recording medium taut between the guide pressure rollers. This prevents undesirable bunching, creasing or deformation caused by slack in the paper between the pressure rollers. In order to achieve this change in the inclination of the axes of rotation, the guide pressure rollers (16/1, 16/2) contain an eccentric axle arrangement.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for the distortion-freeback-and-forth transporting of a recording medium of an imaging machinesuch as a printer or copier between a counter-bearing, which receivesthe recording medium, and a plurality of adjacently arranged pressurerollers which press the recording medium against the counter-bearing.

In electrographic high-speed printers operating with continuous paper,stacking devices are used for stacking the printed continuous paper,such as are described, for example, in DE-C2-26 17 334. For stacking therecording medium on a stacking surface, a transporting device isarranged above the stacking surface, which transporting device comprisesa transporting roller and a pressure roller between which the recordingmedium is transported by friction.

In order to achieve an even contact and guiding of the paper, it is alsocustomary, instead of a continuous pressure roller, to arrange aplurality of individual pressure rollers adjacently, which press therecording medium against the transporting roller. Owing to the design,between these pressure rollers there is a gap in which the paper is notpressed on.

In order that the recording medium, consisting for example of prefoldedcontinuous paper, can be placed securely on the delivery surface in theform of a stack, the recording medium must be fed to the deliverysurface in an unstressed state. This means that the recording mediummust roll along the stack below the feed device so that a zigzag stackcan be formed automatically.

Electrographic printing devices must be constructed in such a way thatthey can be used to print on recording media of the widest variety oftypes, including thin and thick prefolded paper. There is thus the risk,in particular when using relatively thin prefolded paper, that the paperwill form a groove in the region between the pressure rollers whenpassing through the feed device in the stacking device, which makescreasing in the fold and rolling of the paper more difficult. Thisgroove in the paper, as illustrated in FIG. 2, is produced due to thefact that the paper is pushed together between the pressure rollers as aresult of the contact pressure of the pressure rollers.

This groove formation can be avoided if the paper is drawn outwards atthe sides and is thus made taut in the region of the pressure rollers.In order to achieve this, it has already been proposed to mount theouter pressure rollers with a slightly inclined axis in such a way thatthe pressure rollers exert an outward tensile force on the recordingmedium. This transverse force thus produced between the outer pressurerollers makes the recording medium taut in the region located betweenthem.

Electrographic printing devices operate at very high printing speeds of200 sheets per minute and higher. In order that no tearing of therecording medium occurs when the printing device has to be stopped, theprinting device must not be stopped abruptly, but it is braked with aspecific deceleration. The section of the recording medium whichcontinues to be transported during this braking phase must subsequentlybe drawn back again for the next start-up of printing so that printingon the recording medium in the correct position is guaranteed.

During this drawing-back of the recording medium, however, the obliquesetting of the outer pressure rollers now leads to the recording mediumbeing pushed together between the pressure rollers, which can causefaulty stacking at the next start-up of printing. This isdisadvantageous particularly in the case of frequent start-stopoperation.

This problem also occurs in other printing devices which operate, forexample, with large-format single sheets, and in which it is necessaryto move the recording medium back and forth in a printing ortransporting channel.

An electrographic printing device for two-sided printing of singlesheets is known from U.S. Pat. No. 4,953,846. For this purpose, thesingle sheet is firstly printed in the printing station on the front, isfed via a separate return channel to a turning device, is turned there,is laterally offset by means of an obliquely set guide roller and isthen printed on the back. A transporting device is used to transport thesingle sheets, which transporting device comprises a motor-driventransporting roller and a pressure roller which presses the singlesheets resiliently against the transporting roller. In order that thepressure roller can follow the deflection of the single sheet, it ismounted in a bearing body so as to be horizontally displaceable with itsaxis of rotation loose at one end. In this way, the pressure roller isprevented from exerting guiding forces on the single sheets.

An object of the invention is to design a guide pressure roller for arecording medium, in friction-contact with the guide pressure rollers,of a printer or copier in such a way that, on the one hand, the angle ofthe axis of rotation of the guide pressure roller relative to thetransporting direction of the recording medium changes in dependence onthe direction of rotation of the guide pressure rollers and, in sodoing, guide forces are exerted on the recording medium and, on theother hand, the guide pressure rollers is guided precisely.

A further object of the invention is to provide an apparatus for theback-and-forth transporting of a recording medium of a printer or copierbetween a counter-bearing, which receives the recording medium, and aplurality of adjacently arranged pressure rollers which press therecording medium against the counter-bearing, in which no distortion,for example by groove formation, occurs between the pressure rollers.

SUMMARY OF THE INVENTION

The objects are achieved by providing a self-adjusting guide pressureroller which is frictionally contactable against a reversiblytransportable paper. The guide pressure roller has a roller axle with askewed axis of rotation which reversibly self-adjusts to correspond to atransport direction of the paper.

The guide pressure roller includes a roller body contactable against thepaper, the roller axle on which the roller body rotatably bears, and astationary axle on which the roller axle is mounted. The stationary axleextends through the roller axle on a skewed or eccentric axis relativeto a central axis of the roller axle. The roller axle is rotatable onlybetween two stop positions.

A friction moment between the roller body and the roller axle is greaterthan a friction moment between the roller axle and the stationary axleso that the roller axle moves between the stop positions, such that eachstop position is associated with a predetermined orientation of theguide roller bearing. The rolling axis of the roller body is maintainedin an orientation slightly nonperpendicular to the paper transportdirection so that the guide pressure roller frictionally tightens thepaper.

In an embodiment, the stop positions are defined by two projectionsextending from one end of the roller axle. The stationary axle ismounted to an attachment element, and the projections are contactableagainst the attachment element at the respective stop positions.

According to an aspect of the present invention, the guide pressureroller is arranged in an imaging machine, such as a printer or copier.The machine includes a drivable feed roller for reversibly moving thepaper selectively along a forward and backward transport direction. Atleast one, and preferably multiple, conventional pressure rollers arecontactable against the paper to pressing the paper against the feedroller. Each of the conventional pressure rollers has a fixed andnoneccentric axis of rotation. At least one guide pressure rollerfrictionally is contactable against the paper and also presses the paperagainst the feed roller.

In an embodiment, the imaging machine includes at least one cooperatingpair of the guide pressure rollers. The guide pressure rollers of thepair are arranged near opposite edges of a side of the paper. The guidepressure rollers of the pair have oppositely oriented roller bearingssuch that the guide pressure rollers frictionally tighten the paperbetween each other. In another embodiment, the paper has one edge withperforations along which the paper is guided. A guide pressure roller islocated opposite the edge perforations for tightening the paper.

In the transporting apparatus according to the invention, guide pressurerollers are used as outer pressure rollers, whose angular settingrelative to the transporting direction of the recording medium dependson the direction of rotation of the pressure rollers. The recordingmedium is thus made taut during the forward and backward movement of therecording medium and groove formation is avoided.

The guide pressure rollers are mounted on a roller axle as bearingelement. The roller axle, in turn, contains an oblique internal bore inwhich a rigid axle is inserted, about which the roller axle can rotatethrough a particular angular amount between two stop positions. Thearrangement thus forms a type of eccentric.

In order that the guide pressure rollers automatically assume theirangular position producing transverse force, the friction moments of thebearings are selected such that the friction moment between the actualguide pressure roller and the roller axle is greater than the frictionmoment between the roller axle and the rigid stationary axle. During arotary movement of the guide pressure roller, the roller axle is thuscarried along between the stops into the defined stop position. Thisdistribution of friction moments is achieved by a difference in thediameters of the bearing points.

The invention makes reliable stacking of the recording medium possiblein a simple manner, even in start-stop operation.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the detailed description of thepresently preferred embodiments and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated in the drawings and will bedescribed in greater detail below by way of an example.

FIG. 1 is a schematic isometric view of a stacking device for anelectrographic printing device operating with continuous paper;

FIG. 2 is a schematic plan view of a possible groove formation betweentwo pressure rollers in the prior art;

FIG. 3 is a schematic side view of the apparatus according to theinvention in a central position of the guide pressure rollers;

FIG. 4 is a sectional view of a guide pressure roller in a centralposition with a section perpendicular to the transporting direction ofthe recording medium;

FIG. 5 is a schematic side view of the apparatus in an annular positionof the guide pressure rollers, which angular position corresponds to theforward transporting of the recording medium;

FIG. 6 is a front elevational view of the guide pressure roller on theright-hand side in an angular position corresponding to FIG. 5;

FIG. 7 is a schematic side view of the apparatus in an angular positionof the guide pressure rollers, which angular position corresponds to thebackward transporting of the recording medium;

FIG. 8 is a front view of the guide pressure roller on the right-handside in a functional position corresponding to FIG. 7; and

FIG. 9 is an isometric illustration of a roller axle which receives aroller body of the guide pressure roller.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A printing machine (not illustrated here in detail) operating on theprinciple of electrophotography, such as is known, for example, fromEP-B1-04 66 691, uses a prefolded web 10 of continuous paper asrecording medium. After printing and fusing, the recording medium 10 isstacked in a stacking device (FIG. 1) at the outlet of the printingdevice. This stacking device contains a stationary receiving surface onwhich the stack 11 is stacked in the form of a zigzag web, a recordingmedium feed arrangement 12, which changes its position in dependence onthe stack height, feeding the recording medium 10 to the delivery table.The feed arrangement contains a motor-driven feed roller 13 whichextends over the width of the recording medium and can consist, forexample, of hard rubber. The recording medium 10 is guided with its backover this feed roller 13, the feed roller 13 serving as counter-bearing.The recording medium 10 is pressed against the feed roller 13 by meansof a multiplicity of free-running pressure rollers 14 which are arrangedadjacently with a spacing A on a holding device in the form of a holdingstrip 15. Each of the pressure rollers 14 is mounted on a leaf spring 17(FIGS. 6 and 8) which, in turn, is attached to the holding strip 15 bymeans of screws or other attachment elements. The two outer pressurerollers 16/1 and 16/2, disposed at the edge regions of the recordingmedium 10, are designed as guide pressure rollers which change theirangular setting relative to the transporting direction of the recordingmedium in dependence on the transporting direction. They are describedbelow. The central pressure rollers 14 are mounted in a conventionalmanner so as to be freely rotatable on an axle which is firmly clampedbetween the arms of the leaf springs 17.

In normal operation (FIG. 1) of the printing device, the recordingmedium 10 is fed over the feed roller 13 to the stack 11 in thedirection of the arrow. If only conventional rigidly arranged pressurerollers are used as pressure rollers 14 in accordance with the prior artillustration of FIG. 2, a groove 18 or slack bunched area forms on therecording medium 10 between the pressure rollers 14; this groove impedesthe stacking and leads to faulty stacking, since the paper and thus alsothe paper fold are distorted.

If the electrographic printing device is stopped, it is necessary, toavoid the tearing of the recording medium, to brake the paper evenly andnot to stop it abruptly. When continuing the printing operation, it istherefore necessary to return the recording medium 10 to its originalprinting position, for which reason the recording medium is fed back, byreversing the direction of rotation of the feed roller 13, over aparticular section which depends on the braking path. In this case, too,a groove formation 18 would again occur in the case of a rigidarrangement of the pressure rollers 14.

In order to avoid this, according to the invention the two outerpressure rollers 16/1, 16/2 are designed as angularly movable guidepressure rollers 16/1 and 16/2.

A guide pressure roller 16/2 illustrated in section in FIG. 4 has aroller body 19 as the actual paper running roller which can be injectionmolded, for example from plastic material. The roller body 19 is mountedon an integral roller axle 20, illustrated in perspective in FIG. 9. Itconsists of a cylindrical metal or plastic body with an outer surface asa sliding bearing 22 for the roller body 19, the outer surface and thusthe sliding bearing 22 extending parallel to a paper guiding surface 23on the roller body 19. On one side of the roller axle 20, a guide edge 9is formed as a contact surface for the roller body 19 and, on the otherside, a recess 8 for receiving a securing ring 21.

The roller axle 20 contains an oblique internal bore 24 which extendsthrough roller axle 22 obliquely in an eccentric position. Arranged inthis internal bore is a rigid stationary axle 25 (only the axis of whichis indicated in FIG. 9) with assigned bearing bushes 26 for rotatablyreceiving the roller axle 20. The rigid axle 25 is of integral design.The axle 25 is attached to the leaf spring 17 at one end by means of anattachment screw 27 and, at the other end, by means of a clamping devicecomprising a holding block 28 with an associated clamping screw 29. Therotary movement of the roller axle 20 is limited by two upper and lowerprojections 30/1 and 30/2 (FIG. 3, FIG. 9) which are arranged laterallyon the roller axle 20 and interact with stop surfaces 31 arranged on theholding block 28.

In this case, the central position of the bearing arrangement and thusthat of the guide pressure roller are illustrated in FIGS. 3 and 4, inwhich the projections 30/1 and 30/2 are spaced equally from the stopsurfaces 31 of the holding block 28. In this central position, the guidepressure-roller arrangement has a position of the axis of rotation whichcorresponds to the position of the axis of rotation of the central rigidpressure roller 14. It runs perpendicular to the transporting directionof the recording medium and is denoted in FIGS. 6 and 8 by the referencenumeral 32.

The eccentric arrangement of the roller axle 20 and rigid axle 25 is nowselected such that, starting from the central position of the axis ofrotation 32, the angle of the axis of rotation changes in dependence onthe direction of rotation of the roller body 19 and thus in dependenceon the transporting direction, and thus the position of the roller body19 changes relative to the recording medium.

If, as illustrated in FIGS. 5 and 6, the recording medium 10 movesdownward in the stacking direction, the roller body 19 rotates due tofriction in the direction of rotation illustrated in FIG. 5. Startingfrom the central position of the guide pressure-roller arrangement, theroller axle 20 is carried along during this movement since the frictionmoment in the region 22 of the sliding bearing between the roller axle20 and the roller body 19 is greater than the friction moment in theregion of the bearing bushes 26, between the rigid axle 25 and theroller axle 20. As a result of the rotary movement, the projection 30/1is placed against the associated stop surface 31 of the holding block28, and the guide pressure roller (in this case the right-hand guidepressure roller 16/2) assumes the position drawn in FIG. 6, a positionin which, seen in the transporting direction, the axis of rotation 32 isrotated into a position 32/1 of the axis of rotation toward the edges ofthe recording medium.

By means of corresponding, reversed arrangement of the guide pressurerollers and their eccentric arrangement, the guide pressure roller 16/1on the left-hand side is moved into a corresponding rotary-position,likewise directed outward toward the edge of the recording medium, sothat it assumes a position corresponding to the position of FIG. 8. Bymeans of these outwardly directed rotary positions of the guide pressurerollers 16/1 and 16/2, the recording medium is subjected to a transverseforce in the region between the guide pressure rollers 16/1 and 16/2,which makes the recording medium taut in this region and thus prevents agroove formation between the guide pressure rollers 14 and the guidepressure rollers 16/1 and 16/2.

When the direction of the recording medium 10 is reversed by changingthe drive direction of the feed roller 13, the roller body 19 isrotated, due to friction, counter clockwise corresponding to theillustration of FIG. 7. During the rotation, the friction moment betweenthe roller body 19 and the roller axle 20 carries the roller axle 20along until the lower projection 30/2 is placed against the stop surface31 of the holding block 28. The inclination of the axis of rotation hasthus also been reversed, corresponding to the illustration of FIG. 8, sothat the axis of rotation now assumes the position 32/2 of the axis ofrotation. Seen in the transporting direction of the recording medium,the guide pressure rollers 16/1 and 16/2 are again deflected toward theedges of the recording medium. Thus, in this transporting position, too,a transverse force is produced between the guide pressure rollers 16/1and 16/2, which makes the recording medium taut in the region of thepressure rollers 14.

The ratios of the friction moments in the bearings 22 and 26 areimportant for the functioning of this automatic displacement of the axesof rotation of the guide pressure rollers in dependence on thetransporting direction. In this case, it must be ensured that thefriction moment between the roller body 19 and the roller axle 20 isgreater than that between the roller axle 20 and the rigid axle 25. Thisdifference is by virtue of the difference in the diameters of thebearing points.

To avoid the groove formation between the pressure rollers, it issufficient to deflect the guide pressure rollers slightly so that, inthe exemplary embodiment illustrated, the change in the angle of theaxes of rotation, starting from the central position, is only a fewangular degrees, and the spacing between the projections 30/1 and 30/2end the stop faces 31 is thus a few millimeters. The magnitude of thechange in the angle of the axes of rotation depends on the desiredtransverse force to be achieved and the friction moment between theroller body and the recording medium. In dependence on the range ofapplication of the guide pressure rollers, by appropriate choice of theeccentric arrangement inside the roller body 19 and by selection of thestop region, an appropriate adaptation to the desired degree ofdeflection can be achieved.

In the exemplary embodiment illustrated, two guide pressure rollers areassigned on both sides to the edge regions of the recording mediumwithin a stacking device. It is also conceivable to use only one guidepressure roller if, for example, the other side of the recording mediumis guided firmly by means of edge perforations, or even a plurality ofguide pressure rollers if the recording medium involved is a very wideone. It is also conceivable to use the guide pressure rollers for thepurpose of moving, for example, a sheet-like recording medium independence on the transporting direction and to guide it using saidrollers, for example in the context of a single-sheet printing device inthe recording-medium transporting channel or for moving the recordingmedium in the stacking device for each job.

It should be understood that various changes and modifications to thepresently preferred embodiments will be apparent to those skilled in theart. Such changes and modifications may be made without changing thespirit and scope of the present invention and without diminishing itsattendant advantages. Therefore, such changes and modifications areintended to be covered by the appended claims.

List of Reference Numerals

8 Recess

9 Edge, stop

10 Recording medium, prefolded continuous paper

11 Stack

12 Feed arrangement

13 Feed roller, transporting roller

14 Pressure rollers

A Spacing between pressure rollers

15 Holding strip

16/1,16/2 Guide pressure rollers on the left-hand and right-hand sides

17 Leaf spring

18 Groove, distortion

19 Roller body

20 Roller axle

21 Securing ring

22 Sliding bearing, outer surface

23 Guide surface, contact pressure surface

24 Internal bore

25 Rigid axle

26 Bearing bush

27 Attachment screw

28 Holding block

29 Clamping screw

30/1 Upper projection

30/2 Lower projection

31 Stop surface

32 Central position, axis of rotation

32/1 Position of axis of rotation in stacking operation, forwarddirection

32/2 Position of axis of rotation when transported back

What is claimed is:
 1. A guide pressure roller adapted for guiding arecording medium in friction contact with the guide pressure roller, therecording medium bring reversibly movable along a transportingdirection, the guide pressure roller comprising:a roller body; agenerally cylindrical roller axle on which said roller body rotatablybears; and a stationary axle extending through the roller axle in aneccentric position and on which the roller axle is mounted to berotatable in a limited manner between two stop positions at which saidroller axle engages a stationary holding block, a friction momentbetween the roller body and the roller axle being greater than afriction moment between the roller axle and the stationary axle so that,when a rotary movement of the roller body is reversed, the roller axlemoves relative to the stationary axle between the stop positions beforethe roller body moves relative to the roller axle, the guide pressureroller having an axis of rotation which is self-changeable relative tosaid transport direction, each stop position corresponding to arespective orientation of the guide roller bearing on an axis ofrotation which is nonperpendicular to the paper transport direction. 2.The guide pressure roller as claimed in claim 1, having projectionsextending from the roller axle which interact with stop surfaces on anattachment element to which the stationary axle is secured.
 3. The guidepressure roller as claimed in claim 1, further comprising a leaf springelements on which each guide pressure roller is mounted.
 4. An imagingdevice comprising:a mover for moving a recording medium back and forthbetween a counter-bearing and a plurality of adjacently arrangedpressure rollers which press the recording medium against thecounter-bearing; wherein at least the outermost pressure rollers whichrest near opposite edges of the recording medium are a cooperating pairof guide pressure rollers which change their angle of the axes ofrotation relative to the transporting direction to correspond to thetransporting direction of the recording medium, each of said outermostguide pressure rollers including:a roller body; a generally cylindricalroller axle on which said roller body rotatably bears; and a stationaryaxle extending through the roller axle in an eccentric position and onwhich the roller axle is mounted to be rotatable in a limited mannerbetween two stop positions at which said roller axle engages astationary holding block, a friction moment between the roller body andthe roller axle being greater than a friction moment between the rolleraxle and the stationary axle so that, when a rotary movement of theroller body is reversed, the roller axle moves relative to thestationary axle between the stop positions before the roller body movesrelative to the roller axle, the guide pressure roller having an axis ofrotation which is self-changeable relative to said transport direction,each stop position corresponding to a respective orientation of theguide roller bearing on an axis of rotation which is nonperpendicular tothe paper transport direction.
 5. The guide pressure roller as claimedin claim 4, wherein the counter-bearing is a motor-driven transportingroller.
 6. The guide pressure roller as claimed in claim 4, wherein theapparatus is part of a stacking device of an electrographic printingdevice.
 7. An imaging machine comprising:a mover moving the recordingmedium back and forth between a counter-bearing and a plurality ofadjacently arranged pressure rollers which press the recording mediumagainst the counter-bearing, the recording medium being guided on oneside in edge perforations, wherein at least the outer pressure rollerlocated opposite the edge perforations and resting on the recordingmedium is a guide pressure roller which changes its angle of the axis ofrotation relative to the transporting direction corresponding to thetransporting direction of the recording medium, said guide rollerincludinga roller body; a generally cylindrical roller axle on whichsaid roller body rotatably bears; and a stationary axle extendingthrough the roller axle in an eccentric position and on which the rolleraxle is mounted to be rotatable in a limited manner between two stoppositions at which said roller axle engages a stationary holding block,a friction moment between the roller body and the roller axle beinggreater than a friction moment between the roller axle and thestationary axle so that, when a rotary movement of the roller body isreversed, the roller axle moves relative to the stationary axle betweenthe stop positions before the roller body moves relative to the rolleraxle, the guide pressure roller having an axis of rotation which isself-changeable relative to said transport direction, each stop positioncorresponding to a respective orientation of the guide roller bearing onan axis of rotation which is nonperpendicular to the paper transportdirection.
 8. The guide pressure roller as claimed in claim 7, whereinthe counter-bearing is a motor-driven transporting roller.
 9. The guidepressure roller as claimed in claim 7, wherein the apparatus is part ofa stacking device of an electrographic printing device.
 10. A guidepressure roller frictionally contactable against a reversiblytransportable paper, the guide pressure roller having an axis ofrotation which self-adjusts to correspond to a transport direction ofthe paper, the guide pressure roller comprising:a roller bodycontactable against the paper; a roller axle on which the roller bodyrotatably bears; and a stationary axle extending through the roller axleon an eccentric axis relative to a central axis of the roller axle andon which the roller axle is rotatable between two stop positions atwhich positions said roller axle engages a holding block which is fixedrelative to said stationary axle; wherein a friction moment between theroller body and the roller axle is greater than a friction momentbetween the roller axle and the stationary axle so that when a directionof rotation of the roller body is reversed, the roller axle rotates onthe stationary axle between the stop positions before the roller bodyrotates on the roller axle, each stop position corresponding to arespective orientation of the guide roller bearing on an axisnonperpendicular to the paper transport direction which frictionallytightens the paper.
 11. The guide pressure roller as claimed in claim10, wherein the roller axle includes two projections extending from oneend thereof, and wherein the stationary axle is mounted to said holdingblock, the projections being contactable against said holding block atrespective stop positions.
 12. An imaging machine comprising:a drivablefeed roller for reversibly moving a paper selectively forward andbackward transport direction; at least one pressure roller contactableagainst the paper and pressing the paper against the feed roller, eachpressure roller having a fixed axis of rotation; at least one guidepressure roller frictionally contactable against the paper and pressingthe paper against the feed roller, the guide pressure roller having aself-adjustable axis of rotation corresponding to the paper transportdirection, the guide pressure roller having:a roller body contactableagainst the paper; a roller axle on which the roller body rotatablybears; and a stationary axle extending through the roller axle on aneccentric axis relative to a central axis of the roller axle and onwhich the roller axle is rotatable between two stop positions, saidroller axle respectively engaging a stationary holding block at saidstop positions;wherein the friction moment between the roller body andthe roller axle is greater than a friction moment between the rolleraxle and the stationary axle so that when a direction of rotation of theroller body is reversed, the roller axle rotates on the stationary axlebetween the stop positions before the roller body rotates on the rolleraxle, such that each stop position corresponds to a respectivepredetermined orientation of the guide roller bearing on an axisnonperpendicular to the paper transport direction.
 13. The imagingmachine according to claim 12 including at least one cooperating pair ofguide pressure rollers, the guide pressure rollers of the pair beingarranged near opposite edges of a side of the paper, the guide pressurerollers of the pair having oppositely oriented roller bearings such thatthe guide pressure rollers frictionally tighten the paper between them.14. The imaging machine according to claim 12, wherein the paper has oneedge with perforations along which the paper is guided, and wherein aguide pressure roller is located opposite the edge perforations.
 15. Theguide pressure roller as claimed in claim 12, further comprising a leafspring on which each guide pressure roller is mounted.